This invention relates to a servo device for a disc player.
A disc player for playing data recording discs, such as video discs and digital audio discs (hereinafter referred to collectively as "discs", when applicable), essentially includes a tracking servo device for causing a data reading beam of the pickup to follow accurately the recording track, irrespective of eccentricity or other radial irregularity of the disc; and a focus servo device for accurately focussing the data reading beam on the data recording surface of the disk, irrespective of a bend or other depthwise irregularity of the disc.
In the tracking servo device, driving the data reading beam radially on the disc; i.e. a tracking operation, is carried out by a tracking actuator built into the pickup. Driving the objective lens along the optical axis for focusing the light beam; i.e., a focusing operation, is performed by a focus actuator built into the pickup. If the actuators have a small damping ratio, they vibrate, then the servo system may suffer from hunting; that is, the system may be oscillated by the failure of servo convergence or by external interruption. If the servo system oscillates in this manner, then the servo system will not complete its pull-in function. Therefore, it is essential to detect oscillation of the servo system to be able to resume servo convergence.
For this purpose, momentary level of the tracking error signal has conventionally been compared with a predetermined reference voltage (.+-.Vth) by a window comparator and so forth. When the absolute value of the momentary level of the tracking error signal is higher than the reference voltage Vth, it is determined that the servo system is in oscillation, and servo convergence is carried out again. FIG. 5 shows a relationship between the tracking error signal and the reference voltage Vth. In FIG. 5 momentary level in negative polarity side of the tracking error signal is compared with the reference voltage having the absolute level Vth, so as to simplify the explanation.
However, such a method is disadvantageous because some of the discs are larger in error signal level than ordinary discs. In the playing of a disc with a larger error signal, if the absolute level of Vth is low, then, for instance, because of noises as indicated at A in FIG. 5, the servo system may be detected as if it were in oscillation. If the absolute level of Vth is high, then, in the case where a low-pass filter is employed to remove noise components from the error signal, the oscillation as indicated at B in FIG. 5 may not be detected.
As is apparent from the above description, if only the error signal level is used to detect the oscillation of the servo system, then it is very difficult to determine a suitable reference level Vth, and accordingly to detect accurately the oscillation of the servo system.